Deleted app reappeared

without any action on my part?

No, is not possible, you need to search that app.

Use App Library to search the app you are looking for, see -> Organize the Home Screen and App Library on your iPhone -> Organize the Home Screen and App Library on your iPhone - Apple Support           

• If you have find the app, tap & hold on the app, then drag to the Home screen & drop it.

Not sure, see this Apple article: Delete apps on your iPhone, iPad, and iPod touch

Delete apps on your iPhone, iPad, and iPod touch - Apple Support

⤵️

Keep in mind that some built-in Apple apps can't be deleted from your device, so you might not see the option to delete an app. Here’s a list of built-in Apple apps you can delete from your device. If you can't delete an app that's not made by Apple, disable parental controls, then try deleting the app again.

Get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service 

This happened to me last night as well.